As a result of the constantly increasing integration density in the semiconductor industry, photolithographic masks have to project smaller and smaller structures on wafers. In order to fulfill this demand, the exposure wavelength of lithography systems has been shifted to smaller and smaller wavelengths. In the future, lithography systems will use significantly smaller wavelengths in the extreme ultra-violet (EUV) wavelength range (preferably but not exclusively in the range of 10 nm to 15 nm). The EUV wavelength range presents enormous challenges to the precision of optical elements in the optical path of future lithography systems. These optical elements are probably reflective optical elements, since the refractive indexes of presently known materials are essentially one in the EUV range.
EUV mirrors comprise a substrate with a low thermal expansion as for example silica. A multi-layer structure comprising 40 to 60 double layers of silicon (Si) and molybdenum (Mo) is deposited on the substrate which acts as a dielectric mirror. Apart from the substrate and the multi-layer structure, EUV and photolithographic masks or simply EUV masks have additionally an absorber structure which is arranged on the multi-layer structure and which absorbs impinging EUV photons.
As a consequence of the extremely small wavelength already smallest unevenness of the multi-layer structure and minimal deviations of the absorber structure from the predetermined placement and/or of the predetermined target size of the structure elements of the absorber layer result in imaging errors of the wafers illuminated with the EUV mask. Moreover, defects in or on the substrate and/or in the multi-layer structure of the EUV mirror or the EUV mask also result in imaging errors of the structural elements which are imaged by the EUV mask on a wafer which cannot be tolerated. Defects in or on the substrate and/or in the multi-layer structure are in the following called buried defects.
The obvious approach for removing a buried defect would be to remove in a first step the multi-layer structure above the defect and to remove in a second step the uncovered defect and then deposit in a final step the part of the removed multi-layer structure. However, this process cannot be performed due to the multitude of layers of the multi-layer structure and their low thickness of approximately 3 nm for the molybdenum (Mo) layers as well as of approximately 4 nm for the silicon (Si) layers and the high demands on the planarity of the layers or their surfaces.
Rather the U.S. Pat. No. 6,235,434 B1 discloses a method for compensating the amplitude portion of a buried defect by the modification of the absorber structure of an EUV mask close to the buried defect. This process is in the following called “compensational repair” or compensation. FIG. 1 schematically represents its mode of operation. A local decrease of the reflectivity of the surface distorted by the buried defect is compensated by removing portions of the absorber material of neighboring absorber elements.
The above mentioned patent describes that not the geometrical size of the buried defect is compensated, but its equivalent size. The equivalent size of a buried defect depends on its position relative to neighboring absorber elements and the farer the distance is between the defect and the closest absorber element. Phase defects have a smaller equivalent area than amplitude defects. The position and the equivalent size of reflection distortions induced by defects can be determined by a characterization technique as for example photolithographic printing.
The thesis “Simulation compensation methods for EUV lithography masks with buried defects” of C. H. Clifford, University of California, Berkeley, Tech Report No. UCB/EECS-2010-62, http://www.eecs. berkely.edu/Pubs/TechRpts/2010/EECS-21010-62.html proposes two methods to reduce the effects of buried defects when a wafer is exposed. The first method uses pre-estimated design curves in order to determine a modification of the absorber structure solely on the basis of the CD (critical dimension) change induced by the defect. In order to convert the phase error of a buried defect in an amplitude error which can more easily be corrected, and thus reducing the effect of the defect when changing the focus conditions, the second method proposes to cover the defect with absorber material in order to block the light which is reflected from the region of the multi-layer structure distorted by the defect.
The article “Compensation of EUV Multi-layer Defects within Arbitrary Layouts by Absorber Pattern Modification” of L. Pang et al., SPIE Proc. 7969, 79691E (2011) describes the determination of a repairing form for buried defects. The surface contour of the defect is scanned with a scanning force microscope (AFM, Atomic Force Microscope). Using a growth model, the structure of the defect is determined within the multi-layer structure. An aerial image of the defective EUV mask is simulated and the result is compared with the aerial image without defect. The repairing form for the “compensational repair” of the defect is determined pixel by pixel from the comparison.
Another essential aspect of the effective removal of a buried defect is the position of the apparatus used for the defect removal with respect to the localized defect. The repairing can aggravate the negative effects of the buried defect if the position of the buried defect is not precisely taken into account at the defect compensation.
The present invention is therefore based on the problem to provide a method and an apparatus for analyzing a defect and for determining a repairing form of the defect which at least partially avoids the above mentioned drawbacks of the prior art.